Known electrostatic traps (E-traps) employ electrostatic fields for indefinite spatial confinement (trapping) of moving ions, and for arranging highly isochronous ion oscillations. Ions packets are injected into the E-trap field, and ionic oscillation frequencies are detected by an image current detector. Ion mass to charge ratio (m/z) may then determined in calibration experiments since the oscillation frequency F is proportional to (m/z)1/2. Spectra may be reconstructed from signal waveforms by the Fourier Transformation (FT).
U.S. Pat. No. 6,013,913A, U.S. Pat. No. 5,880,466, and 6,744,042, incorporated herein by reference, appear to describe I-path E-traps that employ coaxial ion mirrors for isochronous ion trapping. Ion packets are pulsed injected via a pulsed ion mirror. The described technique appears to suffers low space charge capacity. Injection of more than 1E+4 ions can cause the self-bunching and the coalescence of ion packets.
GB2418528, incorporated herein by reference, appears to describe an I-path ion trap that utilizes radiofrequency fields for radial ion confinement. This system can be impractical due to the combination of limited space charge capacity of the I-path trap and poor image current detection.
U.S. Pat. No. 5,886,346, incorporated herein by reference, appears to describe an orbital trap that uses a cylindrical trap in combination with a hyper-logarithmic electrostatic field. As described, an injection of ions from an external ion source into the electrostatic orbital trap volume is necessarily accompanied by ramping of analytical electrostatic field as ions would otherwise hit trap electrodes. Because ramped potentials conflict with the potential stability, the prolonged injection in combination with the potential ramping causes variations of ion parameters Vs ion mass and, as a result, causes multiple artifacts in spectra. For this reason, optimal parameters have been achieved with pulsed ion injection wherein the duration of injected ion packets is in the order of 100 ns (Makarov et al, JASMS., v. 20 (2009) #8, pp 1391-1396, incorporated herein by reference). This pulsed injection can complicate the formation of ion packets with extended volume and well controlled size, which is desirable for increasing space charge capacity and for minimizing higher harmonic signals.
U.S. Pat. No. 7,994,473, incorporated herein by reference, appears to describe arranging the reciprocal ion motion within a three-dimensional electrostatic field. But to avoid ion losses occurring on the walls within the system, the ion injection also appears to require the ramping of electrostatic potentials. While FIG. 3B appears to prevent side ion excursions along the Z-axis, potential ramping still yields a more limited injection time. Additionally, this potential ramping can tend to affect mass accuracy (parts per million) due to a lower stability of time variable power supply.
In co-pending application WO2011086430, incorporated herein by reference, there is disclosed an extended E-trap which is expected to improve the E-trap space charge capacity of electrostatic traps by orders of magnitude as compared to earlier cited orbital and three dimensional E-traps. The proposed hollow cylindrical geometry can provide significant elongation of the trapping volume without sacrificing the size of the analyzer. But efficiency of the described trap may be limited by pulsed schemes of ion injection.
The speed of spectral acquisition significantly improves when analyzing signals using the Filter Diagonalization Method (FDM) described in Aizikov et al, JASMS 17 (2006) 836-843, incorporated herein by reference. The described method appears to require sinusoidal signals which can lead to an introduction of artifacts and peaks that correspond to higher oscillation harmonics when injecting short ion packets.
Thus, prior art E-traps employ pulsed injection of ion packets accompanied by potential ramping or switching which affects parameters of electrostatic traps. The system described may obviate or mitigate at least one or more of the aforementioned problems and may improve the ion flux throughput and the duty cycle of electrostatic trap mass spectrometers.